During some conditions, turbocharged direct-injection engines may generate increased soot. For example, during some conditions, fuel may not sufficiently mix with air before combustion, for example due to reduced turbulence. Such phenomena may occur when the engine is operating at high speed and/or high loads.
The inventors herein have recognized that one approach to address such an issue may be to utilize a particulate filter in the exhaust. However, regeneration of the filter in the exhaust of a spark-ignited engine aimed at stoichiometric emission control raises numerous issues. As just an example, sufficient excess oxygen for regeneration in the exhaust may result in increased NOx emissions in a three-way catalyst due to saturated oxygen storage in the three-way catalyst.
As such, systems and methods for controlling regeneration of a particulate filter during engine shutdown are provided. However, regeneration during engine shutdown may also cause increased NOx emissions on subsequent engine starts due to oxygen storage saturation. As such, one exemplary method includes, during first engine shutdown conditions, increasing excess oxygen to the particulate filter, and regenerating the particulate filter at least during a portion of engine shutdown. The method further includes, during second engine shutdown conditions (without regeneration, for example), decreasing the excess oxygen to the particulate filter at least during a portion of engine shutdown.
By increasing excess oxygen directed to the particulate filter during some engine shutdowns, the particulate filter can be regenerated during the shutdown while reducing NOx emissions during engine running.
In one particular example, reduced NOx conversion efficiency of a three-way catalyst (or other emission control device in the exhaust stream) during engine shutdown regeneration may have less impact (e.g., less impact on NOx emission control) since the three-way catalyst is not receiving combustion products. Thus, by performing at least some regeneration during engine shutdown, less engine running regeneration may be used. Likewise, particulate filter regeneration can be delayed, reduced, or avoided, during other engine shutdown conditions by reducing excess oxygen during shutting down of the engine. In this way, it is possible to place the three-way catalyst in an improved state (e.g., less oxygen saturation) for a subsequent engine start to better address hydrocarbon and related emissions during the engine start.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.